Lumia 1020 keeps powering off

Question

I've about had it with my Lumia 1020, which keeps randomly turning off. I have to do a soft reset to get it to turn back on, every single time.  The battery level is not the culprit, sometimes it turns off even right after it's finished charging.
I bought the phone in April, it worked fine for the first few weeks, but in early May began shutting down.  That was on WP 8.0 / Lumia Black.  I'd tried a factory reset, the phone was okay for a few days, but then started turning off again. I tried disabling background tasks, still no luck.  My phone updated to 8.1 / Cyan a couple of weeks ago, and the problem still persists.
Again, I've tried blocking apps from using background tasks, even put the phone into battery saver mode always, but no luck.  It turns off whether it's connected to Wi-Fi or cellular data, whether the battery is charged or not.  I always back out of apps, so nothing is left running.
Any other options available? This is really frustrating, especially when I'm missing calls and messages because I haven't realized the phone has shut down.

Answer 1

Yes, as @93tid points out, there isn't much more to do apart from letting Nokia handle it. In some very few cases a faulty sim card can cause weird behavior such as turning the device off, but as you have to reset it to boot after a sudde shut down, thats likely not cause. Probably a hardware failure with the device it self.
Let Nokia care handle it.